Optical pointing devices incorporating a laser illuminated source are already known in the art. Such laser illumination allows optical pointing devices such as mice to work on a much wider variety of surfaces. However the coherent nature of the illumination results in a received image that contains generally high spatial frequencies especially compared to viewing the same surface with a non coherent LED illumination. This high frequency content leads to spatial aliasing due to beyond under-sampled (in the spatial domain). This aliasing leads to several bad effects, such as loss of resolution or apparent “reverse” motion (the “wagon-wheel in motion pictures” effect). One alternative to deal with the higher spatial frequency content is to create smaller pixels (higher spatial capability in the imager). But, two major problems arise with smaller pixels that are a lower mouse-speed/acceleration capability and a lower sensitivity to light (less collection area in the pixel).
U.S. patent application Ser. No. 11/165,537, filed in the name of the same Assignee and which is incorporated in its entirety herein by reference, for instance discloses a method for measuring relative motion between an illuminated portion of a surface and an optical sensing device comprising a coherent light source and a photodetector array, the method comprising the steps of:                illuminating under a determined gradient by means of the coherent light source the surface portion at a determined flash rate;        detecting by means of the photodetector array speckled light intensity pattern of the illuminated portion of the surface for a first flash;        detecting a second speckled light intensity pattern of the illuminated portion of the surface for a second flash;        extracting motion features of two different types from the detected first and second speckled light intensity patterns;        keeping only pairs of neighbouring motion features including one motion feature of both different types;        determining a measurement of the relative motion between the optical sensing device and the illuminated surface portion based on a comparison of kept motion features.        
Although such a solution as disclosed in U.S. patent application Ser. No. 11/165,537 presents several advantages in dealing with a coherent light source in an optical pointing device, since laser illumination has a very large spectral content, there is still a need to further and better control the quantity of data needed to determine a measurement of relative between the optical pointing device and the illuminated surface portion.